This invention relates to a sampler and a method for measuring a waveform of an electric signal repeatedly generated and, in particular, to the sampler and the method which uses a superconductor to measure the waveform with a high time resolution.
In general, a sampler of the type described has been conventionally used to measure a waveform of a current that flow through a wiring on a semiconductor chip of an LSI and the like. In this connection, the current flowing through the wiring may be called a measurement target current while the wiring may be called a measurement target. Such a conventional sampler has been disclosed, for example, in IEEE Transactions on Applied Superconductivity Vol. 9, No.2, June 1999 (page 4081) and is effective to measure the measurement target current caused to flow through the wiring of the LSI that has a complicated pattern and an unknown impedance. The sampler is also advantageous in that the waveform of the measurement target current can be measured without contacting with the measurement target.
More specifically, the conventional sampler has a plurality of Josephson junctions operable in a superconducting state and may be referred to as a superconductor sampler. In this event, the superconductor sampler is supplied with a feedback current controlled in its level during measurement and a trigger current of a pulse shape that defines measurement timing and that is successively varied. Under the circumstances, when the measurement target current is given to the sampler, a sum of the feedback, the trigger, and the measurement target currents is provided or calculated and is compared with a predetermined threshold level. With this structure, it is possible to measure the waveform of the measurement target current by successively varying a value or level of the feedback current and generation timing of the trigger current.
Herein, the superconductor sampler is expected to have a high time resolution and should have generally a bandwidth of more than 100 GHz. In order to observe such a high frequency, such as 100 GHz, it is required that jitters must be reduced to less than several picoseconds. In practice, the trigger current inevitably fluctuates by about 40 picoseconds due to thermal noise. As a result, it has been found out that such a requirement can be never achieved in the conventional superconductor sampler.
In addition, it takes a long time to measure the waveform with a high time resolution and with a high accuracy.